Recently, it has been determined that there are many different members of the myosin superfamily. Most of the characterized myosins are structurally similar to vertebrate skeletal muscle myosin in that they have two globular head domains and a long rod-like domain which self associates to form filaments. These myosins are composed of two heavy chains of 200,000 Da and 4 light chain subunits. Other members of the myosin family may possess distinct structures. In particular, myosin I from Acanthamoeba and vertebrate sources are single-headed and have a lower molecular weight heavy chain than that of skeletal muscle myosin. Chicken intestinal brush border epithelial cells contain a myosin I which has a 100 kDa heavy chain and three calmodulin light chains. This myosin does not form filaments, but does have an actin-activated MgATPase activity and translocates actin filaments in an in vitro motility assay. Motility and actin-activation of the MgATPase activity is inhibited at high calcium concentrations due to a dissociation of a fraction of the calmodulin molecules. Re-addition of calmodulin restores motility and actin-activation of the MgATPase activity. Tropomyosin binding to the actin filaments also inhibits motility and actin-activated MgATPase activity and decreases the affinity of actin for myosin. We have begun the preliminary characterization of a putative myosin I from Nitella axillaris, a freshwater green alga which undergoes vigorous cytoplasmic streaming. Extracts of Nitella translocate actin filaments at a very fast rate in in vitro motility assays. The movement is not inhibited by calcium ions in contrast to the brush border myosin I. We are trying to purify this motor in order to characterize its properties.